1. Field of the Invention
The present invention relates to cable television systems in general and in particular to a scrambled TV-IF signal transmission system for use in cable television systems.
2. Description of the Related Art
Conventional scrambled cable television signal transmission systems, in addition to other signal generating and transmitting components, typically comprise a TV-IF amplitude modulator circuit, a scrambler circuit, a signal combiner circuit and a VSB filter circuit. The TV-IF amplitude modulator circuit is responsive to baseband video and audio signals for providing picture intermediate frequency (PIF) and sound intermediate frequency (SIF) signals. The scrambler circuit is responsive to the baseband video, the PIF and the SIF signals for providing scrambled PIF and SIF signals. The signal combiner circuit combines the scrambled PIF and SIF signals which are then filtered in the VSB filter to provide a scrambled TV-IF signal, i.e. scrambled combined PIF and SIF signals.
Heretofore, the transmission of scrambled TV-IF signals has been limited to local area television sets, generally by amplitude modulated vestigial sideband (VSB) signal coaxial links, wherein they are descrambled and demodulated for use in the sets. To compensate for disparate video and audio signal delays in the VSB filter circuit and to enable the descrambling circuits coupled to the TV sets to function properly, the timing of the scrambled TV-IF signals is .adjusted in the scrambling circuit.
As cable systems expanded, operators used headend stations to transmit scrambled TV signals to hub stations using FM to service remote television sets. When this was done it was found that linear and non-linear distortions of the scrambled composite TV-IF signal in an FM signal transmission link resulted in unacceptable signal quality. This caused the operators of the cable systems to attempt to use the scrambled, but separate, PIF and SIF signals. However, it was found that the timing adjustments made in the scrambling circuit to adjust for the delays in the scrambled TV-IF signal caused by the VSB filter, as described above, necessarily affected the timing of the scrambled PIF and SIF signals at the input to the VSB, thus requiring the use of a suitable delay adjust interface circuit between the headend and hub stations.
The elimination of the need for the above-described delay adjust interface circuit to transmit scrambled TV-IF to a remote hub station together with the timing constraints associated therewith obviously would be of considerable advantage.